Air conditioning systems typically incorporate the standard components of a refrigeration loop to provide chilled water for cooling a designated building space. A typical refrigeration loop includes a compressor to compress refrigerant gas, a condenser to condense the compressed refrigerant to a liquid, and an evaporator that utilizes the liquid refrigerant to cool water. The chilled water is then piped to the space to be cooled.
Air conditioning systems that utilize so called centrifugal compressors are referred to as centrifugal chillers. Centrifugal chillers typically range in size from 100 to 10,000 tons of refrigeration, and are recognized as providing certain advantages and efficiencies when used in large installations such as commercial buildings. The reliability of centrifugal chillers is high, and the maintenance requirements are low, because centrifugal compression involves the purely rotational motion of only a few mechanical parts.
A centrifugal compressor has an impeller that can be thought of as a fan with many fan blades. The impeller typically is surrounded by a duct. The refrigerant flow to the impeller is controlled by variable inlet vanes located in the duct at the inlet to the impeller. The inlet guide vanes operate at an angle to the direction of flow and cause the refrigerant flow to swirl just before entering the compressor impeller. The angle of the inlet guide vanes is variable with respect to the direction of refrigerant flow. As the angle of the inlet guide vanes is varied and the inlet guide vanes open and close, the refrigerant flow to the compressor is increased or decreased. In many applications, the inlet guide vanes are variable ninety degrees between a fully closed position perpendicular to the direction of the refrigerant flow to a fully open position in which the inlet guide vanes are aligned with the refrigerant flow. When the cooling load is high, the guide vanes are opened to increase the amount of refrigerant drawn through the evaporator, thereby increasing the operational cooling capacity of the chiller.
It is important to be able to vary the output capacity of an air conditioning system to meet all conditions of demand in the air conditioned space. At times of high cooling demand, the compressor will run at maximum load or full capacity. At other times the need for air conditioning is reduced and the compressor will run at a reduced capacity. The output of the air conditioning system then is substantially less than the output at full capacity. There is also a need to operate the compressor at the most efficient mode for the capacity that is required at any given time. This is required to reduce the electrical consumption of the air conditioning system to the lowest possible amount for the given load. The most efficient point of operation for a centrifugal compressor is very near a condition known as surge. Operation in the surge condition, however, is undesirable since it is very inefficient and can actually cause damage to the compressor.
In most centrifugal chillers, the compressor is driven by an electric induction motor, either directly or through speed-increasing gears. Because the optimum performance of a centrifugal compressor is strongly influenced by the rotating speed of the centrifugal compressor, much attention has been paid to systems to control the speed of the compressor. Induction motor speed is a function of the frequency of the power supplied to it. An inverter can vary the frequency of the power and thereby control motor speed.
The volume of refrigerant flow through a centrifugal compressor must be adjusted for changes in the load demanded by the air conditioning requirements of the space that is being cooled. Accordingly, a modulating capacity control system is a part of every centrifugal chiller. Control is typically accomplished by varying the inlet guide vanes and the impeller speed, either separately or in a coordinated manner.
The most common method of varying compressor speed is to vary the frequency of the alternating current that is supplied to the induction motor that drives the compressor. As previously indicated, variable-frequency inverters are used to modulate the motor speed.
Control of a centrifugal chiller is typically accomplished by monitoring the temperature of the chilled water as the water leaves the evaporator. The temperature of the water at that point is referred to as the Leaving Water Temperature. The Leaving Water Temperature is an industry wide accepted criteria for establishing control of a centrifugal chiller. The set point of the Leaving Water Temperature, which is the desired operating temperature of the chilled water as the water leaves the evaporator, is selected by the user. U.S. Pat. No. 4,686,834 to Haley et al. is directed to a centrifugal compressor controller for minimizing power consumption while avoiding surge. This patent is assigned to the assignee of the present invention and is incorporated by reference herein.
As discussed above, centrifugal chillers are most efficient when operated near a condition known as surge. At surge, a point is reached where, for the desired cooling output, the pressure differential between the refrigerant immediately at the outlet of the impeller and the pressure of the refrigerant at the inlet of the impeller is large. In this condition, the refrigerant will surge, flowing first backward and then forward through the compressor. This is an unstable operating condition that must be avoided. It is desired to operate the impeller at a speed that is just great enough to avoid the compressor going into the surge condition. This is the lowest speed possible to maintain the compressor in a functional operating condition and meet the cooling requirements. Operating at any faster speed is not efficient from an energy consumption standpoint.
The operating configuration of the compressor which is most efficient for any given capacity is with the inlet guide vanes set to some maximum open position and the rotational speed of the impeller at the lowest possible speed that does not induce surge conditions. In the maximum open position, the variable inlet vanes may still be set at a slight angle with respect to the refrigerant flow direction so that swirl is still imparted to the refrigerant prior to entering the compressor or, alternatively, the vanes may be aligned with the direction of refrigerant flow. The rotational speed of the compressor impeller is controlled by utilizing an inverter that is capable of varying the frequency of the power being supplied to the motor that drives the impeller. Rotational speed of the motor is a direct function of the frequency of the power.
In the past a number of ideas have been advanced in an effort to control centrifugal compressors to achieve high efficiency and yet to avoid surge conditions. U.S. Pat. No. 4,608,833 to Kountz includes a learning mode which alternately incrementally lowers compressor speed and adjusts the position of the prerotational vanes. Once a surge is detected, a current surge surface array is updated and an operating mode is initiated. The initial surge surface array of Table II is generated using minimum Mach number together with a speed correction. However, storage of surge surface arrays for all relevant compressor operating conditions is memory intensive. Additionally, the measurement of the physical position of the inlet guide naves is undesirable in view of the mechanical linkages and economics involved.
U.S. Pat. No. 4,456,618 provides for continual measurement of prerotational vane position, compressor head, and suction flow to calculate an operating point for regulating both the inlet guide vanes and compressor speed. A microprocessor compares the operating point to a prestored surge surface generated by equations comparing compressor head to suction flow rate. If the calculated operating point is too far from the prestored surge surface, the system tries to move the operating point closer to the prestored condition. This system does not determine a region of actual surge based on actual surge events that occur to the specific compressor. It is undesirable to measure the position of the inlet guide vanes and to measure suction flow.
Another existing idea for compressor control is disclosed in U.S. Pat. No. 4,151,725. This method utilizes an inferred compressor head valve to define a control path. Surge avoidance is attempted by deriving a critical Mach number that is a function of compressor head and vane position. The compressor motor is then prevented from delivering an output that is below that critical Mach number. This system generates a surge curve from test data and develops equations to define an operating area that avoids the test surge area. This approach is limited in that it does not account for the actual surge events that occur to the specific compressor over time while functioning in the compressor's unique operating environment.
These methods and others have not proved satisfactory when implemented in the field. The surge point has a certain dynamic that is not accounted for in the previous control methods. Even identically designed compressors have varying surge points under identical operating conditions. Also, over time, the surge points in a given compressor change. Calculated fixed surge points and surge surfaces have not proved the answer to the most efficient operation. For such systems to routinely avoid surge, the operating point must be set artificially distant from the calculated surge conditions since the actual surge conditions unique to the specific compressor are not known. By so setting the operating point, a certain efficiency is sacrificed in the interest of avoiding surge.
The present invention sets forth a control approach developed to improve the efficiency of a centrifugal chiller using a variable speed impeller motor drive. The control methodology was developed with two objectives in mind. The primary control objective is to modulate the compressor capacity to meet the desired chilled water set point. The second objective is to optimize unit efficiency by operating the compressor impeller at the lowest possible speed, while still achieving the desired load capacity and avoiding surge.
Accordingly, it is a general object of the present invention to provide an improved capacity control system of a centrifugal chiller wherein the compressor speed and guide vane position are adjusted to the most efficient operating point while at the same time avoiding a surge condition.
It is an object of the present invention to provide a variable speed capacity control system for a centrifugal chiller wherein the compressor speed and the inlet guide vane position are modulated to meet the chilled water setpoint of an evaporator.
It is an object of the present invention to provide an improved capacity control system of a centrifugal chiller wherein modulating compressor speed and guide vane position is provided in order to place the compressor operating point on an adaptive or dynamic surge control boundary curve.
It is a further object of the present invention to provide an improved capacity control system of a centrifugal chiller wherein the operating point of the compressor is placed on a non-dimensional map generated from sensed centrifugal chiller values.
It is still a further object of the present invention to provide an improved capacity control system of a centrifugal chiller wherein the position of the surge control boundary curve is updated by the detection of real surge conditions that occur over time and is adjusted in response thereto.
It is still a further object of the present invention to provide an improved capacity control system of a centrifugal chiller which does not measure or record an actual surge curve.
It is an additional object of the present invention to provide an improved capacity control system of a centrifugal chiller which operates to prevent or avoid surge using measurements made in the condenser and evaporator.
It is a further object of the present invention to provide an improved capacity control system of a centrifugal chiller which does not require measurements of inlet guide vane position or suction flow rate.